Hemorrhagic shock is a leading cause of death in both civilian and battlefield trauma. Currently available medical monitors provide the capabilities to measure standard vital signs that are often imprecise, subjective, intermittent and inconsistent. More importantly, the appearance of hypotension and other signs and symptoms of shock represent a point in time when it may be too late to apply effective lifesaving interventions. Understanding the physiological signals that provide the best indicators of blood volume loss and impending circulatory failure is critical to bridging the capability gap of identifying the need for early intervention. Under a previous BAMC IRB-approved protocol, we used lower body negative pressure (LBNP) as an experimental model of central hypovolemia to simulate progressive blood loss that results in hemodynamic instability (e.g., hypotension, tachycardia, presyncopal symptoms) in conscious, healthy human subjects. From data collected within this original protocol, we now understand that arterial waveforms (either blood pressure or pulse oximetry) are important variables associated with hypovolemia. Feature extraction and machine learning techniques were applied to these previously collected data and an algorithm was developed called Compensatory Reserve Index (CRI) which was designed to reflect progressive blood loss and decreasing stroke volume. This algorithm was installed in a pulse oximeter in which the photo-plethysmographic waveform (PPG) was used to calculate the CRI. Using this CRI Pulse oximeter, a pretrial study of 24 subjects indicated a high correlation in trending of blood volume loss (DOD study at Mayo Clinic) and a mean correlation of 0.96 when comparing CRI to stroke volume (LBNP at AISR Laboratory). The work proposed herein will validate those findings. During each experiment each test subject will wear 4 FDA cleared pulse oximeters which transmit PPG data to an off-the-shelf handheld tablet (meets military specs) which will calculate and display the CRI value. The subjects will also wear 2 FDA cleared Nexfin hemodynamic monitors that display stroke volume. LBNP will be used to produce progressive central hypovolemia in healthy human subjects until the point of hemodynamic decompensation (presyncope). This approach will validate the correlation of CRI to stroke volume.